Direct fired heater with improved set-up features

ABSTRACT

Features of the direct fired heater can facilitate and improve setup and operation thereof. The direct fired heater can include a housing forming an airflow passage and the airflow passage having an inlet and an outlet; an air moving device to flow air from the inlet to the outlet; gas fired burner disposed in the housing; a pivotal airflow damper disposed in the housing relative to the gas fired burner to control the air flow rate through the burner; and an adjusting device for adjusting the damper from outside the airflow passage. Sensing probes disposed in the airflow passage upstream and downstream of the burner and a pressure differential measuring instrument, such as a manometer, disposed outside of airflow passage in communication with the upstream sensing probe and the downstream sensing probe can measure the differential pressure upstream and downstream of the burner. A shut-off switch also in communication with the sensing probes controls a valve that can interrupt gas flow to the burner when the pressure differential between the upstream and downstream sensing probe is out of a predetermined pressure differential range.

RELATED APPLICATIONS

This application claims benefits and priority of provisional applicationSer. No. 60/660,920 filed Mar. 11, 2005.

FIELD OF THE INVENTION

The present invention relates to a direct fired heater having featuresto facilitate and improve the operational setup of the heater.

BACKGROUND OF THE INVENTION

Heaters for buildings can be either indirect or direct fired. Thetraditional indirect fired heater heats a heat exchanger, such as a tubeor plate, for heating air as the air passes over the exchanger. Unlikean indirect fired heater, a direct fired heater does not have a heatexchanger. The burner in a direct fired heater combusts fuel directlyinto the air stream being heated. The combustion byproducts are releaseddirectly into the air stream, which is then directly discharged into thespace being heated.

Direct fired heaters are preferred over indirect fired heaters forheating large and industrial buildings. In particular, indirect heatershave a low energy efficiency, heat stratification, poor indoor airquality, and require filtration of outside air, dust, and dirt. Incontrast, direct fired heaters are generally about 99.8% energyefficient and provide uniform space temperatures due to a slightpositive pressure that also forces out smoke, odors, and othercontaminates. Direct fired heaters also provide better indoor airquality and are lower in cost to operate when compared to indirectheaters because of the high efficiency of the direct fired system.

Direct fired heaters can be constructed in a variety of configurations.Typically, the burner is arranged upstream of the fan to draw the airthrough the heater, which is a draw-through configuration.Alternatively, a blow-through configuration can be used where the burneris downstream of the fan or blower discharge.

During the operational setup of a direct fired heater, the airflow inthe direct fired heater is adjusted to achieve optimal combustionefficiency. In the past, setup of the heater involved a techniciantaking pressure measurements downstream and upstream from the burnerusing a portable manometer not permanently attached to the heater andmoving the position of a plate inside of the heater in response to thepressure measurements to adjust the airflow. The plate was fastened inan adjusted position inside the heater using screws. This setupprocedure is inconvenient because the technician must stop the operationof the heater to gain access to the interior of the heater, unfasten theplate, adjust its position, restart the heater, take another set ofpressure measurements, turn off the heater, adjust the plate position inresponse to the second set of pressure measurements, and restart theheater. Furthermore, this inconvenient process of adjusting the damperin response to pressure measurement is often repeated until optimalairflow is achieved. The present invention provides a direct firedheater having features that overcome the above disadvantages.

SUMMARY OF THE INVENTION

An embodiment of the invention involves a direct fired heater comprisinga housing forming an airflow passage and the airflow passage having aninlet and an outlet; an air moving device to flow air from the inlet tothe outlet; a gas fired burner disposed in the housing; a pivotalairflow damper disposed in the housing relative to the gas fired burnerto control the air flow rate through the burner; and an adjusting devicefor adjusting the damper from outside the airflow passage. The housingcan have a mixing chamber downstream from the burner for mixingpartially combusted air from the burner and uncombusted air that passesthrough the damper to provide tempered air.

In an illustrative embodiment of the invention, the pivotal damper islocated adjacent to the gas fired burner in the airflow passage and canpivot about an axis transverse relative to the longitudinal axis of theairflow passage. The pivotal damper can include a pivotal shaft havingan end portion residing outside of the airflow passage. The end portionof the pivotal shaft can reside between an interior wall of the housingdefining the airflow passage and an exterior wall of the housing, wherethe exterior wall has an access to the end portion. The pivotal positionof the damper can be adjusted by means of an adjustable arm provided onthe end portion of the shaft.

In another illustrative embodiment of the invention, the direct firedheater comprises a housing forming an airflow passage and the airflowpassage having an inlet and an outlet, an air moving device to flow airfrom the inlet to the outlet, a gas fired burner disposed in thehousing, an airflow damper disposed in the housing relative to the gasfired burner to control the air flow rate through the burner, anupstream sensing probe disposed in the airflow passage upstream of theburner and the damper, a downstream sensing probe disposed in theairflow passage downstream of the burner and the damper, a pressuredifferential measuring instrument disposed outside of the airflowpassage in communication with the upstream sensing probe and thedownstream sensing probe for measuring a differential pressure betweenthe upstream sensing probe and the downstream sensing probe, and anadjusting device to adjust the damper in response to the pressuredifferential measuring instrument. The damper can be a conventionaldamper or a pivotal damper as described above. A mixing chamber can alsobe provided as described above.

In an illustrative embodiment of the invention, the upstream anddownstream sensing probes each can comprise a static or pitot tube. Thepressure differential measuring instrument can be a fluid containingdifferential manometer comprising a U shaped conduit having a firsttubular arm in communication with the downstream sensing probe, a secondtubular arm in communication with the upstream sensing probe, a firstfluid level in the first arm, and a second fluid level in the second armsuch that the fluid levels indicate differential pressure between theupstream sensing probe and downstream sensing probe.

In another illustrative embodiment, a visual indicia is disposed outsideof the airflow passage for viewing in conjunction with the first andsecond fluid levels of the manometer to facilitate adjustment duringoperational setup of the direct fired heater.

In yet another illustrative embodiment of the invention, the directfired heater also includes a shut-off switch in communication with theupstream sensing probe and the downstream sensing probe where the switchinterrupts gas flow to the burner when a pressure differential betweenthe upstream and downstream sensing probe is out of a predeterminedpressure differential range. For example, the shut-off switch can closea valve in a supply line for supplying gas to the gas fired burner.

A method embodiment of the invention comprises a method for controllinga direct fired heater where air is drawn through a passage in a housingand a portion of the air is combusted by a burner comprising the stepsof sensing pressure upstream and downstream from an airflow damper, andadjusting the plate from outside of the passage until the measuredpressure differential upstream and downstream of the damper falls into apredetermined range. In operation of the direct fired heater, gas flowto the burner is shut off when the pressure differential is outside of apredetermined range.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view with portions of the housing broken away toshow the internal features of the direct fired heater.

FIG. 2 is an enlarged, fragmented view of the manometer, shut-offswitch, pivotal shaft, adjustable arm, and a locking means disposed onwall 48.

FIG. 3 is a greatly enlarged, fragmented of the adjustable arm, endportion of the pivotal shaft, and locking means.

FIG. 4 is an enlarged, fragmented schematic view of a burner and damperassembly.

FIG. 5 is an enlarged, fragmented schematic view of a burner, supportmembers for the burner, damper, and air inlet. Interior wall 48 isomitted in both FIGS. 4 and 5 to clearly show the burner and damperassembly.

FIG. 6 is a greatly enlarged, fragmented view of the indicia andmanometer that are be disposed on wall 48.

DETAILED DESCRIPTION OF THE INVENTION

Direct fired heaters can vary greatly and are sometimes consideredcustom built for the application since the heater is configured for thesize of the area to be heated, location of the heater, performancedesired, and componentry selected. A direct fired heater pursuant tothis invention can be mounted upon the roof, adjacent to an exteriorwall, or in the interior of the building to be heated. The outsideambient air flows through the heater, where it is heated, and isdischarged into the area to be heated. A duct can be used to flow theheated air into the building when the heater is remotely mounted fromthe building.

The invention is described below and is especially useful indraw-through direct fired heaters but is not limited thereto as otherconfigurations such as blow-through direct fired heaters can benefitfrom the present invention.

Referring to FIGS. 1-5, an embodiment of the invention comprises adirect fired heater having a housing 8 forming an airflow passage 10 andthe airflow passage 10 having an inlet 12 and outlet 14; an air movingdevice 16 to flow air from the inlet 12 to the outlet 14; a gas firedburner 18 disposed in the airflow passage 10 of the housing 8 togenerate tempered air; a pivotal damper 20 disposed in the housing 8relative to the gas fired burner 18 to control the air flow rate throughthe burner; and an adjusting device 22 disposed on wall 48 to adjust thedamper from outside the airflow passage 10.

In an illustrative embodiment of the invention, the housing 8 can be anelongated enclosure having a top wall 2, bottom wall 3, front wall 4,back wall 5, left side wall 6, and right side wall 7. The housing 8 canbe constructed of aluminized steel walls with a protective finish thatare supported by steel framing 9 although other suitable materials canbe used for the housing. The interior side of the housing walls 2, 3, 4,5, 6, and 7 can form the airflow passage 10 in the housing 8. The inlet12 can comprise an opening across all of wall 5 or a portion thereof.The housing may also include interior walls and exterior walls, forexample, to form a single-wall or double-wall housing.

The inlet 12 can have pretreatment apparatus 44, FIG. 5, for pretreatingthe outside ambient air such. The pretreatment apparatus 44 can be acommercially available screen or filter but is not limited thereto asair conditioning coils or other air treatment apparatuses can be used.For example and not limitation, the pretreatment apparatus 44 is shownin FIG. 5 as a filter that can remove contaminates from the outsideambient air.

Outside ambient air is drawn from the inlet 12 and through the airflowpassage 10. The amount of outside air flowing in the airflow passage 10can range from 100% to 20% by volume with the balance being air from anoptional air return that returns interior air of the room or building.When air from the optional air return is used, the return air is mixedwith the heated air downstream from the burner 18.

In the passage 10, the air is drawn to flow through the gas fired burner18 and the opening 24 adjacent to the burner, for example, as shown inFIG. 1. The air drawn through the burner 18 is used for combustion ofthe fuel by the burner and provides partially combusted air.

The gas fired burner 18 is in communication with a gas supply line 88,which can supply either natural or liquid propane gas or any othersuitable gaseous fuel to the burner 18. The gas flows into the burner 18via the burner manifold 50, FIG. 5. The burner manifold 50 emits naturalor propane gas into an area which is defined by a baffle assemblycomprising an upper baffle 54, lower baffle 56, and side baffles 58, 59.The baffles 54, 56 have a plurality of apertures 52 that allow air frompassage 10 to pass through the baffles 54, 56. The natural or propanegas emitted from the manifold 50 is mixed with air flowing through aplurality of apertures 52 in the upper baffle and lower baffles 54, 56.The airflow through the burner manifold 50 is kept at a constantvelocity during operation of the direct fired heater. The apertures 52function as an air intake for the burner 18 to move air through thebaffles 54, 56 and into the burner 18. The air and gas mixture isignited by an igniter disposed adjacent the burner manifold 50 toprovide partially combusted air. The burner 18 can be a commerciallyavailable fixed-profile, in-line gas fired burner.

For purposes of illustration and not limitation, the gas fired burner 18can be controlled by a commercially available electronic temperaturecontrol system and flame safeguard system although other systems can beused in the practice of the invention. Such electronic temperaturecontrol and safeguard systems are commercially available in direct firedheaters manufactured by AbsolutAire Inc. of Kalamazoo, Mich.

The gas fired burner 18 can be disposed above the damper 20 and mountedto one or more support members 47 as shown in FIG. 5. The supportmembers 47 can be a vertical beam or other suitable support member thatsupports the gas fired burner when the burner is positioned in anopening 24 in wall 46 as shown in FIGS. 4 and 5.

The remainder of the air in airflow passage 10, i.e. the air not drawnthrough the burner 18, is uncombusted ambient air and flows through theopening 24 in interior housing wall 46.

Pursuant to a feature of the invention, a pivotal damper 20 can beprovided to control the air flow rate through the burner 18. The airflow rate increases through the apertures 52 of the burner 18 as thedamper 20 closes relative to opening 24 and the air flow rate decreasesthrough the apertures 52 of the burner 18 as the damper 20 opensrelative to opening 24. For example and not limitation, the damper 20shown in FIG. 5 is horizontally oriented in opening 24 when open andvertically oriented when closed to adjust air flow restriction therein.The damper 20 can reside in opening 24 although the invention envisionsthat the damper can be located at other locations in the housing tocontrol the air flow rate through the burner.

The damper 20 is disposed in the housing 8 adjacent to the burner 18 inthe opening 24 and in the airflow passage 10. The invention envisionsthat two openings can be provided in the wall 46 so that the burner canreside in an opening separate from the damper. The damper 20 can be apivotal damper attached to a pivotal shaft 34 to pivot the damper 20about an axis transverse relative to the longitudinal axis of theairflow passage 10. Shaft 34 can be supported by brackets 38 mounted tothe interior wall 46, which intersects with wall 48. The pivotal shaftincludes an end portion 34 e that can reside outside of the airflowpassage 10 and can be disposed on interior wall 48 or outside of housing8. If a single-walled housing is used, the end portion 34 e extends froma hole in the wall 48 into the space defined by interior wall 48 andexterior wall 7 such that the end portion 34 e resides in the spacebetween interior wall 48 and exterior wall 7. FIG. 1, offered forillustration and not limitation, shows wall 48 broken away with the endportion 34 e residing between interior wall 48 and exterior wall 7.Access to the space between wall 48 and wall 7 can be provided by accessdoors 32, 36 in the outer wall 7.

Referring to FIG. 3, the adjusting device 22 can comprise an adjustablearm 40 on the end portion 34 e to rotate the pivotal shaft 34, which inturn rotates the damper 20, for adjustment. A fastening means 60, whichcan be a nut or other suitable fastener, secures the adjustable arm 40to the end portion 34 e of the shaft 34. The adjustable arm 40 can be anelongated handle that extends from the end portion 34 e for movementforward and backward about the longitudinal axis of the shaft 34. Theadjusting device 22 is not limited to an adjustable arm as other armssuch as, but not limited to, handles, shafts, cranks, levers, or otherdevices suitable for adjusting the damper can be used. The adjustingdevice 22 can adjust the air flow rate through the burner to facilitatethe set-up of the heater.

A locking means 42 can be provided to maintain the position of theadjustable arm 40, which in turn maintains the position of the damper 20in the airflow passage 10. The locking means 42 is disposed on thedistal end of arm 40 and can comprise a wingnut 42 a and a bolt 42 b.The threadable shaft 42 s of the bolt 42 b extends through slot 62 inwall 48 with the head of the bolt located behind the interior wall 48.The wingnut 42 a can be tightened about the shaft 42 s to maintain theposition of the arm 40 or loosened to move arm 40 for adjusting theposition of the damper 20. The slot 62 in wall 48 is arcuate having atrack 64 that extends around the edges of the slot 62. The track 64provides a surface for which the locking means 42 can be secured whilereceived in the slot 62. A visual indicia region 66 is located adjacentthe upper peripheral edge of the track 64 and has a series of markingsto indicate the position of the damper 34 such as shut, ¾, ½, or open inthe opening 24 for adjusting the damper to a position. The orientationof the arm 40, slot 62, track 64, and visual indicia region 66 couldalternatively be downward or generally vertical depending on the heaterconfiguration. With respect to FIG. 1, the adjusting device 22advantageously allows a technician to adjust the damper 20 from outsidethe interior wall 48 so that the inconvenient process of stopping theoperation and accessing the interior of the heater, unfastening theplate, and manually adjusting the position of the plate in the interiorof the heater can be avoided.

In mixing chamber 26, which is downstream of burner 18 and damper 24,the partially combusted heater air from burner 18 and the uncombustedambient air passing through the damper 20 are mixed and provide tempered(heated) air. As shown in FIG. 1 and offered for illustration and notlimitation, the mixing chamber 26 can be formed by walls 6, 46, 48, and49. When air from the optional air return is used, the return air isreceived into the mixing chamber 26 with the uncombusted air.

Air moving device 16 flows the tempered air from the mixing chamber 26to outlet 14 to heat the space that will be heated, typically acommercial or industrial facility. The air moving device 16 can bedisposed in, on, or exterior from housing 8. The air moving device 16can have an air intake 16 a mounted to wall 49, which is shown brokenaway in FIG. 1. The wall 49 can be supported by frames F1 and F2. Forpurposes of illustration and not limitation, the air moving device 16can be a commercially available backward-inclined fan or a squirrel cageblower although other devices that move air can be used in practice ofthe invention. The outlet 14 can be disposed on the bottom wall 3 orother walls such as including, but not limited to, wall 4, 7, 10 of thehousing 8 depending on the configuration of air moving device 16. Whilethe discharged tempered air does contain combustion byproducts, set-upand operation of the heater pursuant to this invention provides airquality that conforms to the American National Standards InstituteZ-83.4 and Z83.18 for indoor air quality.

Pursuant to another feature of the invention, as shown in FIGS. 2 and 6,the direct fired heater can further include an upstream sensing probe80, a downstream sensing probe 82, and a pressure differential measuringinstrument 84 disposed outside of the airflow passage 10 incommunication with the upstream sensing probe 80 and the downstreamsensing probe 82.

The damper 20 in this embodiment can be a pivotal damper as mentionedabove or a single slideable blade, a plurality of louvers, or other typeof conventional airflow damper. In still another embodiment, the dampercan be a motorized damper comprising a motor that is in communicationwith the shaft 34 to move the shaft 34, which in turn moves the positionof the damper 20 in opening 24.

The probes 80, 82 are disposed outside of the airflow passage 10 andeach can be received in an aperture in the interior wall 48 depending onthe configuration of the heater. The upstream sensing probe 80 is incommunication with the airflow upstream of the burner 18 and damper 20and can protrude into the airflow passage 10. The downstream sensingprobe 82 is in communication with the airflow downstream of the burner18 and damper 20 and can protrude into the mixing chamber 26. The probes80, 82 are able to sense any one or a combination of the velocitypressure, total pressure, static pressure, or the air velocity of theairflow in the passage 10. Each probe 80, 82 is preferably aconventional static tube or pitot tube that senses the velocity pressurealthough other types of electrical, mechanical, or pneumatic probes canbe used in the practice of the invention. Furthermore, it is alsoenvisioned that more than two sensing probes can be used to sense theparameters of the airflow in the passage 10.

The pressure differential measuring instrument 84 is communicated toprobes 80, 82 via tubing T1, T2 and displays a reading that correspondsto the pressure differential between upstream sensing probe 80 anddownstream sensing probe 82. The pressure differential measuringinstrument 84 can be a fluid containing differential manometer, as seenin FIGS. 2 and 6, comprising a U shaped conduit 68 having a firsttubular arm 72 in communication with the downstream sensing probe 82, asecond tubular arm 70 in communication with the upstream sensing probe80, a first fluid level 92 in the first arm 72, and a second fluid level90 in the second arm 70. The liquid in the manometer can be water or anyother suitable liquid. For purposes of illustration and not limitation,the pressure differential measuring instrument can be a commerciallyavailable manometer although other pressure differential measuringdevices can be used in practice of the invention.

A visual indicia 86 is located outside of the airflow passage 10 and canbe disposed on wall 48 behind the pressure differential measuringinstrument 84. The indicia 86 has a first predetermined set region 94and second predetermined region set 96 relative to which the first andsecond fluid levels 92, 90 can be adjusted during operational set up ofthe direct fired heater. The technician adjusts the damper 20 to placethe first and second fluid levels 92, 90 in the first and secondpredetermined set regions 94, 96. For example, when the heater isinoperational, the first and second fluid levels 92, 90 will be at themiddle line 99 of the visual indicia 86. When the heater is turned onwithout adjustment, the first and second fluid levels 92, 90 may beoutside regions 94, 96 such as in the air velocity too high regions 100,102 or in the air velocity too low regions 104, 106. The technician isable to use this information provided by the fluid levels 92, 90 and thevisual indicia 86 to conveniently adjust the damper 20 from outside thehousing wall 48. From this information, the technician can adjust thedamper 20 to a position where the first and second fluid levels 92, 90are in the “good” predetermined regions 94, 96 designated on the indicia86 as “good zones” having a green color to provide desired air quality.

Pursuant to still another feature of the invention a shut-off supplyswitch 98 is communicated to sensing probes 80, 82 via tubing T1 and T2to receive measurements therefrom. The switch 98 compares the pressuredifferential between the sensing probes 80, 82 with a predeterminedpressure differential range (i.e. good zones) as calibrated by atechnician for the specific burner and heater configuration. When thepressure differential is outside of the predetermined pressuredifferential range (good zones), the switch 98 sends a signal to a flamerelay 97 that sends a signal to close the valve 87, which interrupts gasflow to the burner 18. The switch 98 and valve 87 can be connected via aconnecting means that can be an electrical, mechanical, or pneumaticconduit sufficient to send a signal from the switch 98 and relay 97 tovalve 87. The connecting means is preferably conventional electricalwires W that receive an electrical signal. The valve 87 has an actuatingmechanism that can be mechanical, electrical, or pneumatic to close andopen the valve in response to a signal from the shut-off switch 98 tostop gas from reaching the burner 18 and can be a conventional solenoidvalve. The invention also envisions that more than one valve 87 can beused in practice of this invention. The supply line 88 is incommunication with a source of gas and can be a conventional pipe forgas having conventional dimensions such as diameters of one inch (1″),one and a quarter inches (1¼′), etc. During operation of the directfired heater, if the pressure differential between the probes 80, 82 isoutside of the predetermined range (good zones) the switch 98 throughthe relay 95 closes the valve 87 to prevent the heater from operatingunder potentially dangerous conditions, i.e. the combustion byproductscould possibly be outside the ANSI standards.

This invention envisions a direct fired heater having one or more of theabove features to facilitate and improve set-up of the heater foroperation to heat a space.

It is to be understood that the invention has been described withrespect to certain specific embodiments thereof for purposes ofillustration and not limitation. The present invention envisions thatmodifications, changes, and the like can be made therein withoutdeparting from the spirit and scope of the invention as set forth in thefollowing claims.

1. A direct fired heater comprising a housing forming an airflow passageand said airflow passage having an inlet and an outlet, an air movingdevice to flow air from said inlet to said outlet, a gas fired burneroperatively associated with the housing, a pivotal airflow damperdisposed in said housing relative to said gas fired burner to controlthe air flow rate through said burner, and an adjusting device to adjustsaid damper from outside said airflow passage.
 2. The direct firedheater in claim 1 wherein said pivotal damper is adjacent to said gasfired burner in said airflow passage.
 3. The direct fired heater inclaim 1 wherein said pivotal damper pivots about an axis transverserelative to the longitudinal axis of said airflow passage.
 4. The directfired heater in claim 1 wherein said housing has a mixing chamberdownstream from said burner for mixing partially combusted air from saidburner and uncombusted air that passes through said damper to providetempered air.
 5. The direct fired heater in claim 1 wherein said pivotaldamper has a pivotal shaft having an end portion residing outside ofsaid airflow passage.
 6. The direct fired heater in claim 5 wherein saidadjusting device comprises an adjustable arm on said end portion torotate said shaft and by which the damper is pivoted, and a lockingmeans to maintain the adjusted position of said adjustable arm.
 7. Adirect fired heater comprising a housing forming an airflow passage andsaid airflow passage having an inlet and an outlet, an air moving deviceto flow air from said inlet to said outlet, a gas fired burner disposedin said housing, an airflow damper disposed in said housing relative tosaid gas fired burner to control the air flow rate through said burner,an upstream sensing probe disposed in said airflow passage upstream ofsaid burner and said damper, a downstream sensing probe disposed in saidairflow passage downstream of said burner and said damper, a pressuredifferential measuring instrument disposed outside of said airflowpassage in communication with said upstream sensing probe and saiddownstream sensing probe, and an adjusting device to adjust said damperin response to said pressure differential measuring instrument.
 8. Thedirect fired heater in claim 7 wherein said upstream sensing probe is astatic tube or pitot tube.
 9. The direct fired heater in claim 7 whereinsaid downstream sensing probe is a static tube or pitot tube.
 10. Thedirect fired heater in claim 7 wherein said pressure differentialmeasuring instrument is a fluid containing differential manometercomprising a U shaped conduit having a first tubular arm incommunication with said downstream sensing probe, a second tubular armin communication with said upstream sensing probe, a first fluid levelin said first arm, and a second fluid level in said second arm.
 11. Thedirect fired heater in claim 10 further comprising a visual indiciadisposed outside of said airflow passage for viewing in conjunction withsaid first fluid level and said second fluid level of said differentialmanometer so that said first and second fluid levels can be adjustedduring operational set up of said direct fired heater.
 12. The directfired heater in claim 7 wherein said damper pivots about an axistransverse relative to the longitudinal axis of said airflow passage.13. The direct fired heater in claim 12 wherein said damper has arotatable shaft having an end portion residing outside of said airflowpassage.
 14. The direct fired heater in claim 13 wherein said adjustingdevice said damper comprises an adjustable arm on said end portion torotate said shaft and by which the damper is rotated; and a lockingmeans to maintain the adjusted position of said adjustable arm.
 15. Thedirect fired heater in claim 7 wherein said housing has a mixing chamberdownstream from said burner and damper for mixing partially combustedair from said burner and uncombusted air that passes through said damperto provide tempered air.
 16. The direct fired heater in claim 7 furthercomprising a supply line for supplying gas to said gas fired burner andhaving a valve to control the gas supply to said gas fired burner, and ashut-off switch in communication with said upstream sensing probe andsaid downstream sensing probe wherein said switch controls said valve tointerrupt gas flow to said burner when a pressure differential betweensaid upstream and downstream sensing probe is out of a predeterminedpressure differential range.
 17. A direct fired heater comprising ahousing forming an airflow passage and said airflow passage having aninlet and outlet, an airflow passage disposed in said housing and havingan inlet and an outlet, an air moving device to flow air from said inletto said outlet, a gas fired burner disposed in said housing, an airflowdamper disposed in said housing relative to said gas fired burner tocontrol the air flow rate through said burner, an upstream sensing tubedisposed in said airflow passage upstream of said burner and damper, adownstream sensing tube disposed in said airflow passage downstream ofsaid burner and damper, a fluid containing differential manometercomprising a U shaped conduit having a first tubular arm incommunication with said downstream sensing tube, a second tubular arm incommunication with said upstream sensing tube, a first fluid level insaid first arm, and a second fluid level in said second arm, a visualindicia disposed outside said airflow passage for viewing in conjunctionwith said first fluid level and said second fluid level of saiddifferential manometer, an adjusting device to adjust said damper inresponse to said differential manometer, a supply line for supplying gasto said gas fired burner and having a valve to control the gas supply tosaid gas fired burner, and a shut-off switch in communication with saidupstream tube and said downstream tube wherein said switch controls saidvalve to interrupt gas flow to said burner when a pressure differentialbetween said upstream and downstream sensing tubes is out of apredetermined pressure differential range.
 18. Method for controlling adirect fired heater where air is drawn through a passage in a housingand a portion of the air is combusted by a burner comprising sensingpressure upstream and downstream from an airflow damper, and adjustingsaid damper from outside of said passage until the measured pressuredifferential upstream and downstream of said damper falls into apredetermined range.
 19. The directed fired heater of claim 5 whereinsaid end portion resides between an interior wall of said housingdefining said airflow passage and an exterior wall of said housing, saidexterior wall having an access to said end portion.